This invention relates generally to heat shrinkable, relatively gas impermeable thermoplastic film which can be heat sealed to itself to form a flexible package. More particularly, it relates to a multilayer plastic film which can be used in the form of a hermetically sealed bag for packaging food products wherein the packaged product is submerged in heated water for cooking. Under these conditions the bag film is shrinkable and retains its integrity.
There are numerous requirements for a cook-in shrink film including: delamination resistance, low oxygen permeability, heat shrinkability representing about 30-50% biaxial shrinkage at about 90.degree. C., and optical clarity.
The prior art has developed a number of multilayer plastic cook-in films and certain of these are in commercial use. For example Bieler et al U.S. Pat. No. 4,104,404 describes a ten layer film comprising four central Nylon layers, an ionomer layer on each outer side of the central layers, and two outer polyethylene layers on each outer side of the ionomer layers. The patentee demonstrates that delamination of the outer polyethylene layer under cook-in conditions was only avoided by irradiation dosage of the entire multilayer film at a level of at least 6 MR.
Another commercially employed cook-in film is the type described in Oberle et al U.S. Pat. No. 4,469,742, also comprising six layers irradiated to dosage of at least 6 MR. The central barrier layer is hydrolyzed ethylene-vinyl acetate copolymer (EVOH), chosen because of its higher softening point and superior oxygen barrier properties as compared to the saran-type barrier layer commonly used in multilayer films for ambient temperature applications. On each side of the barrier layer is an adhesive layer such as chemically modified polyethylene, eg. Plexar. On the outer side of each adhesive layer is a shrink layer such as ethylene-vinyl acetate copolymer (EVA), and the outside (abuse) layer is also the EVA type having a vinyl acetate content of about 5-12 wt. %. The innermost (heat sealing) layer may for example be a propylene-ethylene random copolymer (PER).
It will be apparent from the foregoing that these prior art cook-in films are complex both in terms of multiple materials and sophisticated manufacturing techniques i.e., six layers and relatively high irradiation dosage level to avoid delamination.
A possible solution to the foregoing problems is a three layer film with EVOH as the barrier layer, but tests have shown that such prior art three layer films either would not biorient, would tend to delaminate during cooking or have poor optical properties or have unacceptably low shrink value for cook-in applications, or a combination of these problems. Certain of these problems may be overcome by the use of EVOH-polyamide blend core layers, but three layer films of this type with EVA inner and outer layers have typically been found to delaminate under cook-in conditions. A possible approach to this delamination problem is the use of adhesives, but EVA-EVOH type adhesives would be expected to create blocking problems, i.e., adhesion of adjacent inner EVA layers during manufacture of the film. To avoid the blocking problem, the prior art has provided additional layers on the inside of the adhesion layer or adhesive-containing blend layer, i.e., the aforementioned six layer film.
An object of this invention is to provide an improved cook-in type multilayer plastic film which has less than six layers.
Another object is to provide a three layer cook-in plastic film which is relatively easy to process, i.e., may be readily bioriented.
Still another object is to provide a three layer cook-in type film having high shrink, good optical properties and good inner layer adhesion, and no blocking problem.
A further object of this invention is to provide such a three layer cook-in type film which does not require additives such as processing aids and does not require high irradiation dosage levels on the order of 6 MR to realize high strength.
Other objects and advantages will be apparent from the ensuing disclosure and appended claims.